Problems with scaling meshes. Unwrapping transforms the scale back to normal
It's not that UV unwrapping transforms the scale. It just discards previous UV mapping and creates a new one for selected faces. And it tries to take as much texture as possible (if it won't cause stretching), so in your case it just puts 4 corners of a square face on 4 corners of a square texture. I don't know why you keep unwrapping the mesh after scaling it, but either way you should consider using scale vector of a mapping node: